1. Field of the Invention
The present invention relates to an inertial control method of a wind generator, and more specifically, to a time variant droop-based inertial control method of a wind generator, which can improve the capability of recovering the reduced frequency in the case of a disturbance using a time variant droop coefficient Rvariant calculated using a rate of change of the system frequency.
2. Background of the Related Art
If a disturbance such as tripping of a generator or increase of load occurs in a power system, frequency of a power grid decreases since electrical energy is insufficient. For example, in Korea, if the frequency drops below 59 Hz, an Under Frequency Load Shedding (UFLS) relay operates and sheds load by 6% to prevent consecutive tripping of generators and sheds the load by additional 6% at every 0.2 Hz decrease of frequency. Accordingly, a lowest frequency of the power grid after occurrence of disturbance is an important criterion for determining reliability of the power system, and the frequency of the power grid should not be less than 59 Hz in order to prevent load shedding.
Currently, some of wind generators mainly used for generating wind power performs a Maximum Power Point Tracking (MPPT) control by adjusting the speed of a rotor according to the wind speed in order to generate maximum output power. Since such a Maximum Power Point Tracking control does not consider change of the power grid frequency, significant frequency reduction is inevitable when a disturbance occurs in the power grid. A frequency control function of a wind generator is required to prevent the significant frequency reduction.
Conventionally, methods that contribute the frequency stability of a power grid have been proposed, and one of them is a method of adding a reference value generated by a loop of calculating a rate of change of frequency of a power grid to a reference value of an output for performing a Maximum Power Point Tracking control of a wind generator. Although this method may contribute to suppressing frequency reduction of a power system by temporarily releasing kinetic energy stored in the rotor of the wind generator when a disturbance occurs, it has a limit in that contribution to the recovery of frequency becomes less as time passes after the disturbance occurs.
Meanwhile, in most cases, the amount of power released by an inertial response and droop control of operating synchronous machines after the disturbance is larger than the power capacity of a tripped generator. Therefore, after the frequency rebound, and the sign of the rate of change of frequency becomes negative. Accordingly, this method contributes to the recovery of frequency until the frequency rebounds, however, after the frequency rebounds, output power of a wind power plant decreases due to the inverted sign of the rate of change of frequency, and thus a problem of decreasing the contribution to the recovery of frequency will arise as a result.
Patent document 1 has been disclosed as a conventional technique to solve the problem described above. According to the patent document 1, although it has an effect of further promptly recovering frequency when a disturbance occurs and providing a large amount of power to the power grid, if high frequency components exist in the frequency, a problem will arise that a noise, which gives a bad influence on the control of a wind generator, is amplified due to the high frequency components when a differentiation (or a calculation of a rate of change) is performed on the frequency.